The practice of ellipsometry is well established as a non-destructive approach to determining characteristics of material systems, and can be applied in real time process control. The topic is generally well described in a number of publication, one such publication being a review paper by Collins, titled “Automatic Rotating Element Ellipsometers: Calibration, Operation and Real-Time Applications”, Rev. Sci. Instrum, 61(8) (1990).
In general, modern practice of ellipsometry typically involves causing a spectroscopic beam of electromagnetic radiation, in an imposed, known, state of polarization, to interact with a material system at one or more angle(s) of incidence with respect to a normal to a surface thereof, in a plane of incidence. (Note, a plane of incidence contains both a normal to a surface of an investigated material system and the locus of said beam of electromagnetic radiation). Changes in the polarization state of said beam of electromagnetic radiation which occur as a result of said interaction with said material system are indicative of the structure and composition of said material system. The practice of ellipsometry utilizes said changes in polarization state by proposing a mathematical model of the ellipsometer system and the material system investigated by use thereof, obtaining experimental data by application of the ellipsometer system, and applying square error reducing mathematical regression, (typically), to the end that parameters in the mathematical model which characterize the material system are evaluated so that the obtained experimental data, and values calculated by use of the mathematical model have a “best match” relationship.
A typical goal in ellipsometry is to obtain, for each wavelength in, and angle of incidence of said beam of electromagnetic radiation caused to interact with a material system, material system characterizing PSI and DELTA values, (where PSI is related to a change in a ratio of magnitudes of orthogonal components rp/rs in said beam of electromagnetic radiation, and wherein DELTA is related to a phase shift entered between said orthogonal components rp and rs, caused by interaction with said material system. Said PSI and DELTA are defined by:ρ=rp/rs=Tan(Ψ)exp(iΔ).
As alluded to, the practice of ellipsometry requires that a mathematical model be derived and provided for a material system and for the ellipsometer system being applied. In that light it must be appreciated that an ellipsometer system which is applied to investigate a material system is, generally, sequentially comprised of:                a. a Source of a beam electromagnetic radiation;        b. a Polarizer element;        c. optionally a compensator element;        d. (additional element(s) such as lens(es), beam directing means, and/or windows such as in vacuum chambers);        e. a material system;        f. (additional element(s) such as lens(es), beam directing means, and/or windows such as in vacuum chambers);        g. optionally a compensator element;        h. an Analyzer element; and        i. a Detector System.Each of said components b.–i. must be accurately represented by a mathematical model of the ellipsometer system along with a vector which represents a beam of electromagnetic radiation provided from said source of a beam electromagnetic radiation, Identified in a. above). (Note that elements (a–d) can be referred to a Polarization State Generator (PSG), and elements (f–i) as a Polarization State Detector (PSD)).        
Various ellipsometer configurations provide that a Polarizer, Analyzer and/or Compensator(s) can be rotated during data acquisition, and are describe variously as Rotating Polarizer (RPE), Rotating Analyzer (RAE) and Rotating Compensator (RCE) Ellipsometer Systems.
Where an ellipsometer system is applied to investigate a small region of a material system present, it must be appreciated that the beam of electromagnetic radiation can be convergently entered thereto through an input lens, and, optionally, exit via a re-collimating output lens. It is also possible to have only a collimating lens after the sample. In effect this adds said input, and/or output lens(es) as elements in the ellipsometer system as “additional elements”, (eg. identified in d. and f. above), which additional elements must be accounted for in the mathematical model. If this is not done, material system representing parameters determined by application of the ellipsometer system and mathematical regression, will have the effects of said input, (and output), lenses at least partially correlated thereinto, much as if the input and, (output lenses), were integrally a part of the material system.
It is emphasized that where two sequentially adjacent elements in an ellipsometer system are held in a static position with respect to one another while experimental ellipsometric data is acquired, said two sequentially adjacent elements generally appear to be a single element. Hence, a beam directing element adjacent to a lens can appear indistinguishable from said lens as regards the overall effect of said combination of elements. In that light it is to be understood that present input and output lenses are normally structurally fixedly positioned and are not rotatable with respect to a material system present in use, thus preventing breaking correlation between parameters in equations for sequentially adjacent input and output lenses and an investigated material system by an element rotation technique. While correlation of parameters in mathematical equations which describe the effects of groupings of elements, (such as a compensator and an optional element(s)), can be tolerable, correlation between parameters in the mathematical model of an investigated material system and other elements in the ellipsometer system must be broken to allow obtaining accurate material system representing PSI and DELTA values, emphasis added. That is to say that correlation between parameters in equations in a mathematical model which describe the effects of a stationary compensator and a sequentially next located lens element, (eg. correllation between effects of elements c. and d. or between f. and g. identified above), in a beam of electromagnetic radiation might be tolerated to the extent that said correlation does not influence determination of material system describing PSI and DELTA values, but the correlation between parameters in equations which describe the effects of ellipsometer system components (eg. a., b., c., d., f., g., h. and i.), and equations which describe the effects of a present material system (eg. element e. above), absolutely must be broken to allow the ellipsometer system to provide accurate PSI and DELTA values for said material system. Application of ellipsometry to investigation of a material system present can then present a challenge to users of ellipsometer systems in the form of providing a mathematical model for each of an input and output lens, and providing a method by which the effects of said input and output lenses can be separated from the effects of an investigated material system.
One typical approach to overcoming the identified problem, where space considerations are not critical, and where ellipsometer system configuration can be easily modified, is to obtain multiple data sets with an ellipsometer system configured differently during at least two different data set acquisitions. For instance, a data set can be obtained with a material system present and in which a beam of electromagnetic radiation is caused to interact with said material system, and another data set can be obtained with the ellipsometer system configured in a straight-through configuration, where a beam of electromagnetic radiation is caused to pass straight through an ellipsometer system without interacting with a material system. Simultaneous mathematical regression utilizing multiple data sets can allow evaluation of material system characterizing PSI and DELTA values over a range of wavelengths, uncorrelated with present birefringent retardation effects of present input and output lenses. The problem with this approach is that where ellipsometer systems are fit to vacuum chambers for instance, ellipsometer reconfiguration so as to allow acquisition of such multiple data sets can be extremely difficult, if not impossible to carry out.
Another rather obvious solution to the identified problem is to provide input, and output, lenses which are absolutely birefringence-free, and transparent at all electromagnetic beam wavelengths utilized. That is, provide input, and output, lenses which do not attenuate the magnitude of rp or rs orthogonal components, (or at least do not change their ratio, rp/rs), and which also do not enter phase shift between rp or rs orthogonal components when said beam of electromagnetic radiation is caused to pass therethrough. While control of the effect of a lens on a ratio, (rp/rs), of electromagnetic beam orthogonal components can often rather successfully be accomplished by causing a beam of electromagnetic radiation to approach a surface of a lens along essential a normal to a surface thereof, this is not the case regarding phase shift entered between rp and rs orthogonal components of a said beam of electromagnetic radiation caused to pass therethrough. That is, input, and output, lenses can demonstrate “birefringence”, in that the rp orthogonal component is “retarded” by a different amount than is the rs orthogonal component when said beam of electromagnetic radiation is caused to pass therethrough. To complicate matters, this “birefringent” effect also varies with wavelength and with stresses which can develop in a lens during use because of temperature and physical changes etc.
As described in Parent application Ser. No. 09/162,217, (which is incorporated herein by reference), controlling stress related change is presently achieved with varying degrees of success, where for instance, windows in a vacuum chamber are subject. Windows provided by BOMCO Inc. are produced with the goal of eliminating birefringence, and are mounted in vacuum chambers using copper gasket seals which help to minimize uneven application of stresses and developed strains thereacross. While some success is achieved via this approach, the BOMCO windows are not “perfect” and do demonstrate some remaining birefringence properties, which can vary in unpredictable ways over a period of usage. In addition, BOMCO windows are expensive, costing on the order of $1000.00 each), and are large in size thereby making adaptation thereof to use in a vacuum chamber difficult at times, particularly in retro-fit scenarios. And, there have been cases where BOMCO windows have broken in use. This is highly undesirable as vacuum chambers are often times caused to contain highly toxic and hazardous materials during, for instance, etching and/or deposition steps required in the fabrication of semiconductor devices. Where vacuum chamber windows are the subject, an alternative to use of the BOMCO windows is to simply use standard vacuum chamber windows, which, while significantly less expensive, demonstrate order of magnitude larger birefringence effects. (Note, BOMCO windows provide birefringent effects on the order of approximately six-tenths (0.6) to two-tenths (0.2) degrees over a range of wavelengths of from four-hundred (400) to seven-hundred-fifty (750) nanometers, whereas standard vacuum windows demonstrate birefringent effects on the order of six (6.0) to three (3.0) degrees over the same range of wavelengths). (Note, birefringent retardation typically follows an approximate inverse wavelength, (eg. 1/wavelength), relationship). However, where standard vacuum chamber windows are utilized, compensation of their effects is required. Similar concerns apply where input and output lenses, and associated ellipsometrically indistinguishable ellipsometer system components are concerned.
A need is thus identified for a method of practicing ellipsometry which enables the breaking of correlation between parameters in equations which describe retardance entered to orthogonal components of a beam of electromagnetic radiation caused to interact with a material system, and parameters in equations which describe birefringent effects on said orthogonal components in said beam of electromagnetic radiation caused by input and output windows of a vacuum chamber, and/or by input and output lenses and/or by electromagnetic beam directing means etc.
Various researchers have previously noted the identified problem, where vacuum chamber windows are the topic, and proposed various first order mathematical model equation correction techniques as solution, which approaches have met with various degrees of success where vacuum chamber input and output windows demonstrate on the order of a maximum of two (2) degrees of birefringence. This, however, leaves the problem unsolved where birefringence approaches six (6.0) degrees, as commonly occurs in standard vacuum chamber windows, and can also occur in lens systems, particularly at wavelengths of four-hundred (400) nanometers and below. Thus is identified a problem to which the present invention calibration methodology applies.
Patents and/or Published Applications which describe the use of multiple element lenses in Ellipsometer and the like systems include:
Applications of Danner et al.:
                EP1 172 642 A2;        JAPAN 2002 2098591 A        U.S. 2002/0024669;Applications:        WO 91/14157;        WO 92/12404 by Rudolf Corp.;        WO 96/18205;        WO 99/02950;Patents:        U.S. Pat. No. 4,671,657 to Calvani et al.;        U.S. Pat. No. 5,166,752 to Spanier et al.;        U.S. Pat. No. 5,349,497 to Morris;        U.S. Pat. No. 5,877,859;        U.S. Pat. No. 5,963,327 to He et al.;        U.S. Pat. No. 5,978,087 to Patterson et al.        Japanese Application H6 (1994)-22332.        
Other patents of which the Inventor is aware include those to Woollam et al, U.S. Pat. No. 5,373,359, patent to Johs et al. U.S. Pat. No. 5,666,201 and patent to Green et al., U.S. Pat. No. 5,521,706, and patent to Johs et al., U.S. Pat. No. 5,504,582 are disclosed for general information as they pertain to ellipsometer systems.
Additional Patents of which the Inventor is aware include U.S. Pat. Nos. 5,757,494 and 5,956,145 to Green et al., in which are taught a method for extending the range of Rotating Analyzer/Polarizer ellipsometer systems to allow measurement of DELTA'S near zero (0.0) and one-hundred-eighty (180) degrees, and the extension of modulator element ellipsometers to PSI'S of forty-five (45) degrees. Said patents describes the presence of a variable, transmissive, bi-refringent component which is added, and the application thereof during data acquisition to enable the identified capability.
A patent to Thompson et al. U.S. Pat. No. 5,706,212 teaches a mathematical regression based double fourier series ellipsometer calibration procedure for application, primarily, in calibrating ellipsometers system utilized in infrared wavelength range. Birefringent window-like compensators are described as present in the system thereof, and discussion of correlation of retardations entered by sequentially adjacent elements which do not rotate with respect to one another during data acquisition is described therein.
A patent to Woollam et al, U.S. Pat. No. 5,582,646 is disclosed as it describes obtaining ellipsometic data through windows in a vacuum chamber, utilizing other than a Brewster Angle of Incidence.
Patent to Woollam et al, U.S. Pat. No. 5,373,359, patent to Johs et al. U.S. Pat. No. 5,666,201 and patent to Green et al., U.S. Pat. No. 5,521,706, and patent to Johs et al., U.S. Pat. No. 5,504,582 are disclosed for general information as they pertain to Rotating Analyzer ellipsometer systems. The 359 Patent describes a Rotating Analyzer Ellipsometer (RAE) which can comprise a Collimating Lens prior to the Sample being investigated, but has no Lens after said Sample. While not specifically disclosing a focusing lens before the Sample and no collimating lens thereafter, said 359 Patent does obviate the use of a lens before a sample in an ellipsometer, with no lens after said sample being present.
Patents identified in a Search specifically focused on the use of lenses, preferrably achromatic, in ellipsometry and related systems are:                U.S. Pat. Nos. 5,877,859 and 5,798,837 to Aspnes et al.;        U.S. Pat. No. 5,333,052 to Finarov;        U.S. Pat. No. 5,608,526 to Piwonka-Corle et al.;        U.S. Pat. No. 5,793,480 to Lacy et al.;        U.S. Pat. Nos. 4,636,075 and 4,893,932 to Knollenberg; and        U.S. Pat. No. 4,668,860 to Anthon.        
The most relevant patent found is U.S. Pat. No. 5,917,594 to Norton. However, the system disclosed therein utilizes a spherical mirror to focus an electromagnetic beam onto the surface of a sample in the form of a small spot. Said system further develops both reflection and transmission signals via application of reflective means and of reflection and transmission detectors. The somewhat relevant aspect of the 594 Patent system is that a positive lens and a negative meniscus lens are combined and placed into the pathway of the electromagnetic beam prior to its reflection from a focusing spherical mirror. The purpose of doing so is to make the optical system, as a whole, essentially achromatic in the visible wavelength range, and even into the ultraviolet wavelength range. It is further stated that the power of the combined positive lens and negative meniscus lens is preferrably zero. It is noted that, as described elsewhere in this Specification, said 594 Patent lens structure, positioning in the 594 Patent system, and purpose thereof are quite distinct from the present invention lens structure and application to focus a beam of electromagnetic radiation. In particular, note that the 594 Patent lens is not applied to directly focus and/or recollimate a beam of electromagnetic radiation onto a sample system, as do the lenses in the present invention. And, while the present invention could utilize a meniscus lens in an embodiment thereof, the 594 Patent specifically requires and employs a negative meniscus lens to correct for spherical aberations caused by off-axis reflection from a spherical mirror, in combination with a positive lens to correct for achromatic aberation introduced by said negative meniscus lens. Further, the present invention system does not require reflection means be present in the path of an electromagnetic beam after its passage through the focusing lens thereof and prior to interacting with a sample system, as does the system in the 594 patent wherein a focusing spherical mirror is functionally required.
A patent to He et al., U.S. Pat. No. 5,963,327 is disclosed as it describes an ellipsometer system which enables providing a polarized beam of electromagnetic radiation at an oblique angle-of-incidence to a sample system in a small spot area.
A patent to Johs et al., U.S. Pat. No. 5,872,630 is disclosed as it describes an ellipsometer system in which an analyzer and polarizer are maintained in a fixed in position during data acquisition, while a compensator is caused to continuously rotate.
Patent to Dill et al., U.S. Pat. No. 4,953,232 is disclosed as it describes a rotating compensator ellipsometer system.
Patents co-owned with this application, which patents Claim various Compensator Designs recited in Claims herein, and which patents are incorporated hereinto by reference are:                U.S. Pat. No. 5,946,098 to Johs et al.;        U.S. Pat. No. 5,963,325 to Johs et al.;        U.S. Pat. No. 6,084,674 to Johs et al.;        U.S. Pat. No. 6,084,675 to Herzinger et al.;        U.S. Pat. No. 6,100,981 to Johs et al.;        U.S. Pat. No. 6,118,537 to Johs et al.;        U.S. Pat. No. 6,141,102 to Johs et al.Patents cited in examination of said patents included U.S. Pat. No. 4,556,292 to Mathyssek et al. and U.S. Pat. No. 5,475,525 to Tournois et al.        
A patent to Bjork et al., U.S. Pat. No. 4,647,207 is disclosed as it describes an ellipsometer system which has provision for sequentially positioning a plurality of reflective polarization state modifiers in a beam of electromagnetic radiation. While said 207 Patent mentions investigating a sample system in a transmission mode, no mention or suggestion is found for utilizing a plurality of transmitting polarization state modifiers, emphasis added. Patent Nos. 4,210,401; 4,332,476 and 4,355,903 are also identified as being cited in the 207 Patent. It is noted that systems as disclosed in these patents, (particularly in the 476 Patent), which utilize reflection from an element to modify a polarization state can, that if such an element is an essential duplicate of an investigated sample and is rotated ninety degrees therefrom, then the effect of the polarization state modifying element on the electromagnetic beam effect is extinguished by the sample.
Patents to Rosencwaig et al., U.S. Pat. Nos. 4,750,822 and 5,595,406 are also identified as they describe systems which impinge electromagnetic beams onto sample systems at oblique angles of incidence. The 406 Patent provides for use of multiple wavelengths and multiple angles of incidence. For similar reasons patent U.S. Pat. No. 5,042,951 to Gold et al. is also disclosed.
A patent to Osterberg, U.S. Pat. No. 2,700,918 describes a microscope with variable means for increasing the visibility of optical images, partially comprised of discrete bi-refringent plates which can be positioned in the pathway between an eyepiece and an observed object. Other patents identified in a Search which identified said 918 Patent are U.S. Pat. No. 3,183,763 to Koester; U.S. Pat. No. 4,105,338 to Kuroha; U.S. Pat. No. 3,992,104 to Watanabe and a Russian Patent, No. SU 1518728. Said other patents are not believed to be particularly relevant, however.
A patent, U.S. Pat. No. 5,329,357 to Bernoux et al. is also identified as it Claims use of fiber optics to carry electromagnetic radiation to and from an ellipsometer system which has at least one polarizer or analyzer which rotates during data acquisition. It is noted that if both the polarizer and analyzer are stationary during data acquisition that this patent is not controlling where electromagnetic radiation carrying fiber optics are present.
A patent to Chen et al., U.S. Pat. No. 5,581,350, is disclosed as it describes a method for regression calibration of ellipsometers which is very much similar to that disclosed earlier in an article by Johs.
Patent to Wang. et al., U.S. Pat. No. 6,587,282 is disclosed as it describes a three lens system with specific curvature and spacings associated with each of the lenses.
Patent to Uhrich et al., U.S. Pat. No. 6,829,049 is disclosed as it describes a broadband ellipsometer with all refractive optical system for focusing a probe beam onto a sample.
As present invention preferred practice is to utilize a spectroscopic source of electromagnetic radiation with a relatively flat spectrum over a large range of wavelengths U.S. Pat. No. 6,628,917 to Johs is disclosed. Patents relevant thereto include U.S. Pat. No. 5,179,462 to Kageyama et al. is identified as it provides a sequence of three electromagnetic beam combining dichroic mirrors in an arrangement which produces an output beam of electromagnetic radiation that contains wavelengths from each of four sources of electromagnetic radiation. Each electromagnetic beam combining dichroic mirror is arranged so as to transmit a first input beam of electromagnetic radiation, comprising at least a first wavelength content, therethrough so that it exits a second side of said electromagnetic beam combining dichroic mirror, and to reflect a second beam of electromagnetic radiation, comprising an additional wavelength content, from said second side of said electromagnetic beam combining dichroic mirror in a manner that a single output beam of electromagnetic radiation is formed which contains the wavelength content of both sources of electromagnetic radiation. The sources of electromagnetic radiation are described as lasers in said 462 Patent. Another patent, U.S. Pat. No. 5,296,958 to Roddy et al., describes a similar system which utilizes Thompson Prisms to similarly combine electromagnetic beams for laser source. U.S. Pat. Nos. 4,982,206 and 5,113,279 to Kessler et al. and Hanamoto et al. respectively, describe similar electromagnetic electromagnetic beam combination systems in laser printer and laser beam scanning systems respectively. Another patent, U.S. Pat. No. 3,947,688 to Massey, describes a method of generating tuneable coherent ultraviolet light, comprising use of an electromagnetic electromagnetic beam combining system. A patent to Miller et al., U.S. Pat. No. 5,155,623, describes a system for combining information beams in which a mirror comprising alternating regions of transparent and reflecting regions is utilized to combine transmitted and reflected beams of electromagnetic radiation into a single output beam. A patent to Wright, U.S. Pat. No. 5,002,371 is also mentioned as describing a beam splitter system which operates to separate “P” and “S” orthogonal components in a beam of polarized electromagnetic radiation.
Various papers were also identified as possibly pertinent, and are:
A paper by Johs, titled “Regression Calibration Method for Rotating Element Ellipsometers”, Thin Solid Films, 234 (1993) is also disclosed as it describes a mathematical regression based approach to calibrating ellipsometer systems.
A paper by Nijs & Silfhout, titled “systematic and Ramdom Errors in Rotating-Analyzer Ellipsometry”, J. Opt. Soc. Am. A., Vol. 5, No. 6, (June 1988), describes a first order mathematical correction factor approach to accounting for window effects in Rotating Analyzer ellipsometers.
A paper by Kleim et al, titled “Systematic Errors in Rotating-Compensator ellipsometry”, J. Opt. Soc. Am., Vol 11, No. 9, (setp. 1994) describes first order corrections for imperfections in windows and compensators in Rotating Compensator ellipsometers.
Other papers of interest in the area by Azzam & Bashara include one titled “Unified Analysis of Ellipsometry Errors Due to Imperfect Components Cell-Window Birefringence, and Incorrect Azimuth Angles”, J. of the Opt. Soc. Am., Vol 61, No. 5, (May 1971); and one titled “Analysis of Systematic Errors in Rotating-Analyzer Ellipsometers”, J. of the Opt. Soc. Am., Vol. 64, No. 11, (November 1974).
Another paper by Straaher et al, titled “The Influence of Cell Window Imperfections on the Calibration and Measured Data of Two Types of Rotating Analyzer Ellipsometers”, Surface Sci., North Holland, 96, (1980), describes a graphical method for determining a plane of incidence in the presence of windows with small retardation.
A paper by Jones titled “A New Calculus For The Treatment Of Optical Systems”, J.O.S.A., Vol. 31, (July 1941), is also identified as it describes the characterizing of multiple lens elements which separately demonstrate birefringence, as a single lens, (which can demonstrate reduced birefringence).
A paper by Zapien et al., titled: “Real-Time Spectroscopic Ellipsometry from 1.5 to 6.5 eV”, Thin Solid Films 364, (2000), shos lenses on both sides of a sample.
A paper by Li titled: “Flying Height Measurementon Al203 Film of a Magnetic Slider”, J. or Tribiology, (October 1997) describes a 17 micron spot size achieved by a focusing lens.
A paper by Ghazzawi et al., titled: “Spectroellipsometry Characterization of Directly Bonded Silicon-On-Insulator Structures, Thin Solid Films 233 (1993).
Finally, a paper which is co-authored by inventors herein is titled “In Situ Multi-Wavelength Ellipsometric Control of Thickness and Composition of Bragg Reflector Structures”, by Herzinger, Johs, Reich, Carpenter & Van Hove, Mat. Res. Soc. Symp. Proc., Vol. 406, (1996) is also disclosed.
Even in view of relevant prior art, there remains need for ellipsometer systems which comprise input, and optionally output, lenses that allow focusing spectroscopic electromagnetic beams as small spots on material substrates. Further, in view of the inability of first order corrections to break birefringence based correlation between input and/or output lenses and a material system, there remains need for a second order mathematical model equation correction technique which enables breaking correlation between a material system characterizing DELTA and in-plane retardance entered to a beam of electromagnetic radiation by input and output lenses through which said beam of electromagnetic radiation is caused to pass. This is particularly true where lens birefringent retardance exceeds a few degrees. In addition, need remains for a lens system, and the method of its construction, which enables very precise lens characteristics realization
The present invention provides a system with the identified attributes.